It has been known that many organs are associated with a series of movements of a human in masticating and swallowing food taken into his or her mouth (eating and swallowing movements). In particular, teeth, a jaw, cheeks, lips, and a tongue are closely related to mastication of food. If the functions of organs are impaired by, for example, disorders or any other possible causes, what is important is to examine and measure the impaired functions of the organs so as to determine whether the eating and swallowing movements are well performed, or to what degree the movements are done. If rehabilitation of eating and swallowing functions is carried out, it is also important to study the effect of the rehabilitation.
The function of the lips, among the above-listed organs, can be measured by a commonly known method of measuring a force closing upper and lower lips (i.e., a lip closing force). For example, Patent Document 1 discloses a method of measuring the lip closing force. According to this method, a tip end portion of a long and narrow, plate-shaped tool, provided with a pressure sensing part having a pressure sensor, is disposed between the upper and lower lips, and a portion of the tool with the pressure sensing part is sandwiched between the upper and lower lips to measure the lip closing force.